Color flow mapping (CFM) is one approach to estimate and visualize the flow inside blood vessels in a combined B and CFM duplex mode. Either the CFM image covers the complete field of view equal to the B-mode image or the user places a box using the graphical user interface (GUI) of the ultrasound system to indicate a region of interest (ROI). Unfortunately, CFM only shows relative blood flow information of whether the flow is towards or away from the transducer. Hence, it cannot show absolute flow direction or absolute velocities.
For quantitative information about blood flow, spectral Doppler (D) imaging has been used with B mode and CFM imaging. Generally, spectral Doppler imaging displays information about the power spectral density of the received blood flow Doppler-shift frequencies in the form of a real-time updating of a 2D spectrogram image. The display simultaneously displays a B-mode image and a D-mode information along with the spectrogram. The spectrogram has been defined by Doppler frequency-shift along the y-axis and time along the x-axis. The intensity of each pixel in the spectrogram represents the signal power and the value range has been indicated by a color or gray-scale bar.
Spectral Doppler imaging with automatic velocity curve tracing has been used to measure a mean velocity and a maximum velocity as a function of time. Generally, automatic velocity curve tracing automatically measures these parameters and plots the measurements via curves on the display, e.g., overlaid on top of the spectrogram. Information such as the nature of the blood flow (e.g., its complexity, level of retrograde-flow, level of turbulence, etc.) can be indirectly obtained through recognizing the look of the spectrogram and the sound of the Doppler audio produced by the scanner. The user may also measure the volume flow through the vessel.
By way of example, the user first scans the blood vessel of interest with the probe in B mode and CFM mode. Next, the user activates D mode with automatic velocity curve tracing. The user then places the Doppler gate at the blood vessel. While positioning the probe to get a scan-plan through the blood vessel of interest, the user places an axial scan-line with a Doppler gate across the vessel. The spectrogram is then formed from data received from this Doppler-gate. To get velocity information along the y-axis in the spectrogram instead of Doppler-shift frequency, the user has to set a beam-to-flow angle by indicating the general direction of the blood vessel and flow.
This angle has been less than or equal to sixty degrees (60°) for acceptable precision in the resulting velocity estimations. The user then adjusts the Doppler gate size to cover a cross-section of the vessel. The user then steadies the probe and acquires data for at least two heart beats. The user then freezes or pauses scanning and activates volume flow estimates. The user then manually measure the cross-section or diameter of the vessel on the B-mode image where the Doppler gate is located, and the system calculates an estimate of the volume flow based on the mean velocity curve trace for a user selected section and of the spectrogram and shows the result in a measurement area.
Unfortunately, the above process concurrently using B mode, CFM mode and D mode imaging can be complex and tedious.